Electromagnetic interference can be defined as undesired conducted or radiated electrical disturbances from an electrical or electronic source, including transients, that can interfere with the operation of other electrical or electronic apparatus. Such disturbances can occur at frequencies throughout the electromagnetic spectrum. Radio frequency interference (“RFI”) is often used interchangeably with electromagnetic interference (“EMI”), although RFI more properly refers to the radio frequency portion of the electromagnetic spectrum usually defined as 10 kilohertz (KHz) to 100 gigahertz (GHz).
Electronic equipment is typically enclosed in a housing. The housing not only serves as a physical barrier to protect the equipment from the environment, but also can serve to shield EMI/RFI radiation. Enclosures having the ability to absorb and/or reflect EMI/RFI energy may be employed to confine the EMI/RFI energy within the source device, and to insulate the device or other external devices from other EMI/RFI sources. To maintain accessibility to the internal components, most enclosures are provided with openable or removable accesses such as doors, hatches, panels, or covers. Gaps typically exist between the accesses and the corresponding mating surfaces that reduce the efficiency of the electromagnetic shielding by presenting openings through which radiant energy may be emitted. Such gaps also present discontinuities in the surface and ground conductivity of the housing, and in some cases may generate a secondary source of EMI/RFI radiation by functioning as a slot antenna.
For filing gaps between the mating surfaces of the housing and removable accesses, gaskets and other seals are used to maintain electrical continuity across the structure, and to exclude environmental degradants such as particulates, moisture, and corrosive species. Such seals are bonded or mechanically attached to one or both of the mating surfaces and function to establish a continuous conductive path by conforming to surface irregularities under an applied pressure.
Conventional processes for manufacturing EMI/RFI shielding gaskets include extrusion, molding, and die-cutting. Molding involves the compression or injection molding of an uncured or thermoplastic resin into a certain configuration. Die-cutting involves the forming of a gasket from a cured polymeric material, which is cut or stamped into a certain configuration using a die. Form-in-place (“FIP”) processes are also used for forming EMI/RFI shielding gaskets wherein the process involves the application of a bead of a viscous, curable, electrically-conductive composition in a fluent state to a surface that is subsequently cured-in-place by the application of heat, atmospheric moisture, or ultraviolet radiation to form an electrically-conductive, EMI/RFI shielding gasket.
Electrical conductivity and EMI/RFI shielding effectiveness is typically imparted to polymeric gaskets by incorporating conductive materials within the polymer matrix. The conductive elements can include metal or metal-plated particles, fabrics, meshes, and fibers. The metal can be in the form of, for example, filaments, particles, flakes, or spheres. Examples of metals include copper, nickel, silver, aluminum, tin, and steel. Other conductive materials that are used to impart EMI/RFI shielding effectiveness to polymer compositions include conductive particles or fibers comprising carbon or graphite. Conductive polymers such as polythiophenes, polypyrroles, polyaniline, poly(p-phenylene)vinylene, polyphenylene sulfide, polyphenylene, and polyacetylene may also be used.
In addition to shielding EMI/RFI radiation, in certain applications it is also desirable that the seal be transparent to incident broad spectrum radiation used for detection, location, or recognition purposes. For example, microwave radiation from 5-18 GHz, 35 GHz, 94 GHz, 140 GHz and 220 GHz has useful military significance. Electromagnetic radiation incident on a surface will be partly reflected and partly absorbed by the material and the sum of these effects determines the shielding effectiveness. The shielding effectiveness depends on several factors including the frequency of the electromagnetic radiation, the conductivity of the shielding material, the thickness and permeability of the shielding material, and the distance between the radiating source and the EMI/RFI shield. At high frequencies, above about 10 GHz, shielding effectiveness is primarily determined by the ability of the shielding material to absorb the incident radiation. Ferromagnetic particles with high permeability such as iron, carbonyl iron, cobalt metal alloys, and nickel metal alloys are used as radar absorbing materials.
In addition to providing continuous electrical conductivity and EMI/RFI shielding effectiveness, in certain applications it is desirable that gasket or seals to surfaces exposed to the environment, such as in aviation and aerospace vehicles, not lead to corrosion of the metal surfaces. When dissimilar metal and/or conductive composite materials are joined in the presence of an electrolyte, a galvanic potential is established at the interface between the dissimilar conductors. When the interfacial seal is exposed to the environment, particularly under severe environmental conditions such as salt fog or salt fog containing a high concentration of SO2, corrosion of the least noble of the conductive surfaces will occur. Corrosion may lead to a degradation in the EMI/RFI shielding effectiveness of the seal. Mechanisms other than galvanic potentials, e.g. crevice corrosion, may also compromise the electrical and mechanical integrity of the enclosure.
Polysulfide polymers are known in the art. The production of polysulfide polymers is characterized by Fettes and Jorzak, Industrial Engineering Chemistry, November, 1950, on pages 2,217 to 2,223. The commercial use of polysulfide polymers in the manufacture of sealants for aerospace applications has long been known and commercially used. Polysulfide sealants have been used to seal aircraft exterior fuselage because of the high tensile strength, high tear strength, thermal resistance, and resistance to high ultraviolet light. Polysulfide sealants have been used to seal aircraft fuel tanks because of the resistance to fuel and adhesion upon exposure to fuel.
Polysulfide sealants are generally applied by extrusion using a gun. Extruding a sealant to seal apertures in airframe such as those associated with access doors or panels can require a significant amount of effort. The interior perimeter of the access door opening is masked and the exterior perimeter of the access door is coated with a release agent to avoid sealing an access door shut. The sealant is extruded and the access door is put in place and clamped down to force the excess sealant around the access door. The sealant is allowed to cure and the excess sealant is trimmed away. This process is time intensive and can add significant labor demands for servicing aircraft with many access doors. Some aircraft can have as many as a hundred or more access doors that are used to cover sensitive electronic equipment or fittings that must be periodically accessed.
Accordingly, it is desirable to provide compositions and methods for sealing access doors, for example those in an airframe of an aviation or aerospace vehicle, that are not as labor and time intensive as the conventional extrusion method for sealing the access doors. It is also desirable to provide such compositions and methods that further provide effective EMI/RFI shielding and cause minimal corrosion to conductive surfaces.